When you've completed the game, you can start a new playthrough with developer commentary enabled. If you haven't beaten the game, be warned that some commentary may spoil puzzle solutions.

In this chamber, you'll find commentary nodes from:

[Paul Graham] Because our test chamber environments were simplified for training purposes, we created visual hot spots within the rooms to guide players' attention. The design is essentially a balance between round objects and sharp objects, the sharp objects representing background elements and the round objects - such as doors and moveable props - comprising our points of visual interest.

[David Kircher] For the first few months of development, we rendered the views through portals to two offscreen textures. This approach was easy to implement and was compatible with a wide range of graphics hardware. Unfortunately, this method was incompatible with antialiasing and consumed a large amount of video memory in order to handle recursive views through several portals. Because of these disadvantages, we switched to a system which renders portal views recursively into the frame buffer with the aid of the stencil buffer to isolate pixels corresponding to a given portal. This is a more effective scheme because it is compatible with antialiasing and does not consume any additional video memory for offscreen textures.